The Global Positioning System (GPS) is now well-known in the field of navigation. A brief description of the GPS will however be provided with reference to FIG. 1 where the earth rotates about its north-south axis 16 in the manner shown. The GPS constellation is made up of a plurality of twenty four (24) satellites 18 placed in six planes equally spaced about the earth. In each plane 20 will be three or four satellites more or less equally spaced in the plane. Each satellite in the plane is arranged to exhibit a nearly-circular orbital path about the earth at an altitude of about 20,000 kilometers. Each plane 20 is disposed in angular and spatial relation to each of the other five planes thereby forming an angle of approximately 60 degrees between adjacent planes. At the same time, each plane 20 is disposed in angular relation to the equatorial plane to form an angle of about 55 degrees therebetween. The time required for a satellite to travel in its planar orbit about the earth is approximately twelve sidereal hours.
The primary electrical power for each satellite is provided by solar cells and rechargeable batteries. Only during eclipses are the batteries used exclusively. The satellites transmit a pair of encoded signals at predetermined L-band frequencies at L.sub.1 : 1575.42 megaHertz (MHz) and L.sub.2 : 1227.60 MHz. One reason for utilizing a pair of signals from each satellite is to compensate for the adverse effects of the ionosphere on the determination of the range. The transmitted signal frequencies are a multiple of the 10.23 MHz primary frequency reference contained in each satellite. This frequency reference may be either a Cesium or Rubidium atomic frequency standard. The signals transmitted by the satellites consist of two codes: the coarse code C which repeats every millisecond, and the precise code P which repeats each week. Attached to each of these codes is the navigation message, a 50 bits/second data stream containing the predicted satellite ephemeris and clock corrections and other information. Each GPS satellite is assigned its own unique code by which it is identified; this is called Code Division Multiple Access. The user's receiver can select the satellites to track by selecting the appropriate code numbers and generating the corresponding code sequence.
The GPS is being developed as a world-wide, all weather, space based navigation system for military and civilian applications. For most users, operating under quiescent conditions, the current generation of GPS receivers and controlled radiation pattern antennas are adequate. However, when conditions include high multipath environments, high radio frequency interference due to close proximity to radar, field of view blockage due to a ship's structure, a rolling and pitching platform during severe weather, and/or jamming, the low power level of the GPS signals causes current GPS receivers to operate at a reduced efficiency.